Farm worker infections with MRSA — the first numbers

Since the first identification in 2004 of MRSA ST398, also known as “pig MRSA” or livestock-associated MRSA (archives of posts here and here), that drug-resistant organism has been found being carried asymptomatically by farm workers and veterinarians, and causing illness in health care workers, hospital patients, and people with no known ties to agriculture. One of the persistent data gaps, though, has been whether farm workers themselves have been made sick by it.

It’s a difficult question to answer for a nested set of reasons: First, in most of the states, MRSA (methicillin-resistant Staphylococcus aureus, or drug-resistant staph) is not a reportable disease; that is, a doctor who diagnoses it in a patient is under no obligation to tell any public health authority about that patient’s case. And second, the testing required to distinguish livestock-associated MRSA from community-acquired or hospital-acquired is not something that primary-care medical personnel have access to; you have to go to a state laboratory or an academic medical center to do the appropriate molecular typing. Those tests are expensive to perform, and their results primarily are useful to public health, not to individual medical practitioners. So finding out where that nascent epidemic is going has been unusually challenging.

Writing in the Annals of Agricultural and Environmental Medicine, , Kerry Leedom Larson, Tara Smith and Kelley Donham of the University of Iowa College of Public Health say that, in a self-reported survey, 3.7 percent of farm workers have been diagnosed by a doctor with any kind of MRSA infection.

The team approached 800 pork producers (with the assistance of the National Pork Board, which funded the study) and asked them to complete a survey that described their farms, the workers’ jobs, and the medical history. In two rounds of soliciting, they received 135 completed surveys from the 783 who indicated they were still farming (17.2 percent). Of those responses, five (3.7 percent) indicated physician-diagnosed MRSA infections, and four (2.9 percent) indicated their farms housed pigs that had been diagnosed with antibiotic-resistant skin infections.

So, what do those numbers mean?

There is surprisingly little data about the rate of MRSA infections in the United States. The commonly repeated numbers, such as “1.5 percent of the US population in 2004,” refer to colonization or symptomless carriage, of which infection is a much smaller subset. The other numbers that get bandied about are MRSA infections as a percentage of all staph infections — 80 percent in San Francisco ERs, for instance — but those, again, don’t say anything about the rate of infection in the population overall.

I asked Tara Smith (who, disclosure, appears in my book Superbug) what she thought. ‘We can’t draw any good conclusions, because we had a low response rate and this is all self-reported,” she told me. “We can’t say if these are ST398 or not, or acquired on the farm or not. But given all those caveats, it seems high to me.”

Some interesting notes and important qualifiers:

While all the farms were conventional (not organic or free range/grass-fed), most of them were on the smaller side: 47 percent raised fewer than 10,000 hogs per year and 13 percent had fewer than 50 sows or produced fewer than 1,000 finish hogs. But 43 percent of the US market is dominated by just 27 operations that each raise at least 500,000 per year. Those farms — which are not in this survey — operate under the kind of conditions in which antibiotic-resistant bacteria are most likely to arise.

Farms are naturally concerned about biosafety, and a number of them appeared to be following practices that would make any worker infections less likely, such as offering showers (71.1 percent), soap (65.2 percent) and farm-only clothing (58.5 percent). But those good practices may be failing in the details: Only 34.8 percent had a cleaning schedule for their showers; did farm-worker laundry (35.6 percent daily, 4.4 percent weekly), which prevents any organisms from the farm contaminating the home; or had a policy for taking care of wounds (34.1 percent). Only 4.4 percent had had their barns tested for the presence of resistant bacteria.

Workers might be putting themselves at risk of infection also, following behaviors that make athletes, for instance, more vulnerable to MRSA infection: they shared clothing (35.6 percent) or boots and socks (41.5 percent), towels (48.1 percent) and soap (54.1 percent).

So what does this tell us? It’s a beginning. It makes clear that MRSA is occurring among farm workers, probably more than in the general population, and it spotlights some ways in which they are being made more vulnerable to infection from animals or from each other. But as the researchers note:

The number of MRSA infections reported here may be underestimated. Pork producers may not seek medical treatment, or infections may be misdiagnosed by rural physicians. Some producers may not want to disclose MRSA infections in workers or pigs due to fear of identification… we cannot de-termine whether livestock-associated or human-associated strains are the cause. Future collaboration with rural physicians could provide clinical samples from pork production workers and enable molecular typing to occur.